This invention relates to plastic contact lenses in which a small portion, for example, 1 to 1.5 mm, of an outer chord of the lens is not present. This is called truncation. Truncated plastic contact lenses are per se known and are used to ensure stability of the lens on the eye of the wearer, whereas round contact lenses are allowed to rotate freely on the layer of tears carrying same on the eye.
Correction of astigmatism requires a toric-shaped lens which must be held in position and stabilized from significant rotation to achieve the desired degree of correction. Toric contact lenses are thus often provided with truncation, either single or double in which two opposed parallel chords are made in the lens. The conventional procedure used to form a truncated lens is to prepare a complete toric lens then manually remove a small portion of the lens along the appropriate periphery to provide the truncation. Such manual operations are not only labor-intensive, but there is also the opportunity of introducing striations into the lens which must be removed by further manual operation.
Finished lenses are manually truncated in the following manner: the finished lens, if in the wet state, must be dried then, in a hand operation, the lens is guided on a cutting tool such as a grinding wheel, and the desired chord-shaped area is removed. With a toric lens further steps are necessary--the lens is inserted in a Vertexometer and the major axis of the toric surface is determined. Then the optimum toric lens-toric cornea relationship is determined with respect to the major axes of the toric lens-toric cornea surfaces to correct the astigmatic refractive error of the eye. This positional relationship is noted on the lens and while retaining such relationship the lens is truncated so that the resulting truncated lens will settle in a corrective position on the corneal surface with the truncation or chord-like area proximal to the lower eyelid. In approximately forty-five percent of the patients requiring toric contact lenses the major axis of the toric cornea is horizontal, i.e., 180.degree..+-.5.degree.. A toric lens with a single truncation rides on the cornea with its major axis "coinciding" with the major axis of the toric cornea and with the truncated area of the lens in a downward position, i.e., the truncation is considered to be parallel to both the major axis and the lower eyelid. The truncation step is followed by buffing the truncated edge in a conventional manner. Truncation of contact lenses, therefore, is time-consuming and labor-intensive.
This invention relates to molded toric contact lenses, either mono- or bitruncated, in which the finished lens is cast in a single operation in a novel mold assembly. Bitruncated lenses have the flat side of the truncations parallel to each other. In a single truncated lens the truncation will be "parallel" to the lower eyelid. During blinking of the eye the eyelid passes over the length of the truncated chord of the lens in a coinciding fashion rather than an acute angle which would tend to rotate the lens in the eye.
In a preferred aspect, this invention also relates to procedures and to molds for fabricating truncated plastic contact lenses having at least one toric surface and preferably including both prism ballast and cylinder. Lenses so produced exhibit outstanding positional stability and comfort to the wearer.
Toric contact lenses were first introduced by Stimpson in 1950 while the toric base contact lens was described in 1952 and 1953 by Schapero and Baglien. Prior art procedures for the manufacture of toric contact lens are time-consuming and fairly expensive and involve a considerable amount of hand finishing. Moreover, the apparatus used to fabricate such lenses are often elaborate and difficulty is encountered to achieve reproducible results. Illustrative patents include U.S. Pat. Nos. 3,079,737; 3,344,692; 3,482,906; 3,535,825; 3,623,800 and 3,950,082. Truncated lenses are conventionally made by removing a chord from the edge portion of a finished contact lens.
Casting methods are known for preparing contact lenses in order to reduce or eliminate costly machining and polishing of the lens; see U.S. Pat. No. 4,121,896 to T. H. Shepherd. According to the process described in this United States patent, complementing replica male and female mold members each having an optical molding surface are prepared by injection molding of a plastic composition in the master mold pairs. The plastic mold members are then longitudinally aligned and used to mold contact lenses, one of the molding surfaces being provided with a flexible circumferential rim which defines the periphery of the resulting lens. The lens-forming material which will constitute the lens is charged into the female mold member or portion. The complementing male member or portion is brought into contact with the female mold member so that the flexible rim portion is seated on the opposite molding surface. During molding the lens-forming material contracts causing the flexible rim to flex, inwardly usually, whereby the two mold members move towards each other. Once the molding cycle is completed the mold members are separated and the resulting contact lens is removed in a condition ready for use, except for cleaning and perhaps a buffing of the lens edge.
A toric lens comprises two basic parts, namely, a cylinder, to correct astigmatism of the eye, and means such as prism ballast and/or truncation, preferably both, for ensuring positional stability when placed on the cornea. Non-astigmatic lenses do not have such a requirement and the lens is allowed to rotate freely upon the layer of tears carrying it. Toric lenses, on the other hand, require proper, rather precise positioning in the eye. One procedure to achieve lens stability is to physically remove a small portion, e.g., 1 to 1.5 mm, of an outer chord of the contact lens (generally about 14 mm in diameter) from the finished lens. This is called truncation. Opposed parallel clippings are called double truncations.
Another means of achieving positional stability to a toric contact lens is to provide a somewhat larger mass of plastic material at or proximate an edge of the lens so that, in use, when the lens rotates this heavier edge will gravitate towards the bottom of the axis of rotation, i.e., the lower eyelid, and serve to stabilize the lens on the eye. This is known in the art as prism ballast. Preferably both prism ballast and truncation are employed.
A method for molding toric contact lenses is described in U.S. Pat. No. 4,208,365 to R. J. LeFevre in which, like the Shepherd patent, a mold system comprised of a pair of mold members is used to cast the lens, the desired toric optical surface being achieved by distorting the optical molding surface of one of the flexible mold members used to mold the lens. Distortion of the member is induced by tensioning means such as a torque ring surrounding the mold part to achieve the desired configuration. The mold system containing lens-forming material is then closed by positioning the mold members in lens-forming relationship with each other, the principal longitudinal axis of curvature of each mold member being in alignment. This casting technique produces the cylinder feature of a cast toric contact lens but lacks significant prism ballast.
A method for casting toric contact lenses having a prism ballast is described in copending application Ser. No. 199,780, filed Oct. 20, 1980 in the name of Eric Noble in which a mold assembly is comprised of lens-forming mold members: a first mold member having an optical concave toric molding surface, a second mold member having an optical convex molding surface, the principal longitudinal axis of curvature of each of said molding surfaces forming an angle .theta. when said mold members are in lens-forming relationship with each other. This procedure produces a prism ballast toric-shaped contact lens, which may be either hard or flexible, by casting in a closed mold system which compensates for shrinkage that occurs during polymerization or vulcanization of the lens-forming material used to make the lens. This process results in a prism ballasted toric lens that exhibits significant positional stability in the eye.
Molds made from thermoplastic materials and useful for casting contact lenses are described in U.S. Pat. Nos. 4,121,896 and 4,208,364 both to T. H. Shepherd and U.S. Pat. No. 4,208,365 to R. J. LeFevre; however as excellent as they are these molds do not deliver truncated contact lenses with prism ballast. Truncation requires a separate manual step.
The toric contact lenses made in accordance with the Noble procedure are stated to possess significant positional stability when placed in the eye of the wearer. While this is a meritorious advance in terms of casting a prism ballasted toric contact lens, lens rotation of upwards to approximately 10.degree. from the normal can occur on the eye of the wearer. That is, the prism ballast feature of Noble's toric lens generally prevents the lens from rotating more than about 10.degree. from its normal position (the position where the prism ballast is, so to speak, proximal to the lower eyelid).
In addition to the degree of rotation of the contact lens on the eye of the wearer the time required to return the lens to its "normal" position is also an important consideration. For the astigmatic wearer the period of time the lens is other than in the normal position represents time the wearer has imperfect vision. We have found that when truncated lenses made as herein described are properly placed to correct astigmatism, the return to normal time, when the lens is rotated out of position, is surprisingly short.
The replica mold member with a continuous flexible rim about both the circular section and the "straight line" chord section as well are readily fabricated employing conventional injection molding techniques using metal master mold parts and thermoplastic resins, e.g., polypropylene. A description of the metal master mold parts is described hereinafter.
One of the objects of the present invention is to provide a replica mold that will faithfully produce a truncated plastic contact lens in virtually finished form. Also included in our invention are methods of making such molds, preferably of plastic, for regular, toric and toric/ballasted lenses. Truncated lenses so produced will exhibit excellent positional stability of less than approximately 4.degree. average rotation from the norm and under optimum conditions lenses are obtained in which the rotation is no greater than approximately 2.degree. in a majority of the cases. As truncated lenses are normally associated with the need to stabilize the lens in one position, as for instance the correction of corneal and refractive astigmatism, lenses so produced are normally toric and preferably ballasted as well, although the present invention need not be so restricted.
Another object of our invention is to cast a truncated plastic contact lens, either rigid or flexible, preferably a toric, ballasted lens in a single molding operation so that the thus cast lens requires no or virtually no manual finishing and in which truncation is effected in a manner that retains the corrective astigmatic relationship of the resulting toric lens on the toric cornea of the wearer.
Another object of the invention is to provide a truncated, cast contact lens with prism ballast with exceptional positional stability rotating, if at all, only a few degrees from the normal position on the eye of the wearer.
These and other objects of the invention will be apparent from the following more detailed description.